In an MR system, it is necessary that a magnetic field gradient be produced along each of the three mutually orthogonal axes (X-, Y-, and Z-axes) of a Cartesian coordinate system. Coils for producing a Z-axis magnetic field gradient in a system where a static magnetic field is generated in the Z-axis direction have been heretofore fabricated by winding wires on a bobbin.
In this prior art technique, annular grooves are formed in given positions on a bobbin, and wires are wound a given number of times in the grooves in order to produce a desired magnetic field gradient. This method needs skillfulness to fabricate the coils. Also, it is difficult to machine this coil assembly.
A method of fabricating Z-axis gradient coils by forming a desired coil pattern on a flexible insulative substrate, winding the substrate around a bobbin, and bonding the substrate to the bobbin has been proposed as disclosed in U.S. Pat. No. 4,910,462 entitled "Etched Z-Axis Gradient Coils For NMR System". These coils need neither the prior art bobbin-machining step nor the prior art wire-winding step. Also, it is easy to fabricate these Z-axis gradient coils, because it is only necessary that the flexible insulative substrate be wound on and bonded to the bobbin.
Where a strong magnetic field gradient is produced using the Z-axis gradient coils disclosed in the above-cited U.S. Pat. No. 4,910,462, it is necessary to increase the number of conductive strips existing in unit length of coil by reducing the width of each conductive strip contained in the coil pattern. Where the conductive strips are narrowed in this way, the electrical resistance of each conductive strip and hence the electrical resistance of the whole coil is increased. Generally, to cause an electrical current to flow through a coil of large electrical resistance, a power supply of a large capacity is necessary. Furthermore, heat generated by the coil presents problems. Therefore, coils having smaller electrical resistance are preferred.